The use of a non-azeotropic mixture in the heat pumps and cooling machines is known in order to improve the thermal capacity or the performance coefficient of the plant. The application of non-azeotropic mixtures has been the object, in particular, of U.S. Pat. Nos.: 4,089,186, 4,344,292 and 4,406,135.
It has been discovered that the use of such mixtures is of particular advantage when the non-azeotropic mixture is only partly vaporized in the evaporator, the completion of the vaporization being achieved by heat exchange with the compressor unit. As a matter of fact, the compressor unit constitutes a heat source generally at a higher temperature than the external fluid cooled in the evaporator; thus, a mixed working fluid which vaporizes according to a temperature profile, may reach, by heat recovery from the compressor unit, a temperature, at the end of the boiling period, higher than that obtained when proceeding to a complete vaporization in the evaporator. The process thus makes it possible to reduce the compression rate of the plant. It also offers the advantage of cooling the gases during the compression step, with the favorable effects of a reduction of the compression work and of a decrease of the discharge temperature.
The selection of the compressor type in a cooling machine or a heat pump essentially depends on its power. Generally, reciprocating compressors are used for suction rates, by volume, lower than 1000 m.sup.3 /h; screw compressors for volume rates up to 5.000 m.sup.3 /h and, for higher rates, turbocompressors are substituted to positive-displacement compressors.
Within the smaller power ranges, hermetic units are widely used. They comprise a steel jacket or bell formed of two welded parts, with extending suction and delivery pipes. The compressor is generally of the reciprocating type but may be also of the rotary type.
The hermetic units have many advantages: protection against external agents, compactness, low acoustic level, low price. The main problem of their use is the high temperature of the gases. As a matter of fact, the sucked gases cool simultaneously the lubricating oil and the motor coils. The design of the compressor itself is thus responsible for a substantial overheating of the gas before admission to the feed values and for a high discharge temperature.
Now, the compression power is the higher as the suction temperature is high. On the other hand, a too high thermal level is a factor of wear of the motor and may limit the condensation temperature as a result of the liability of coolant and lubricant decomposition. In fact, the compression of a gas in a hermetic compressor unit may be considered as substantially adiabatic since all the evolved heat is recovered by the working fluid at the delivery.
The accompanying figures illustrate embodiments of the invention without limiting the scope thereof.